1. Field of the Invention
The present invention relates to a milk protein product, and more particularly to a milk protein product having water-dispersible and hydrophilic properties.
2. Related Art Statement
Sodium caseinate is milk protein which has been known as having excellent water-dispersible and hydrophilic properties.
Although sodium caseinate is easily dissolved in water to exhibit strong adhesion, it has off flavour due to alkali and contains a large amount of binding sodium since it is produced by processing the milk protein with a strong alkali, such as sodium carbonate.
For this reason, addition of sodium caseinate to foodstuff is limited, to avoid excess intake of sodium, generally to a level of about 0.2 to 0.5 wt%. It is necessary that an orally-administered liquid diet must contain 3 to 5 wt% of protein. However, the necessary content of protein cannot be supplied by the only use of sodium caseinate. Accordingly, soybean protein is used together with sodium caseinate to prepare such an orally-administered liquid diet at the present time.
Milk protein products, other than sodium caseinate, known in the art include acid casein, rennet casein and co-precipitation milk proteins. Acid casein is prepared by adding an acid to milk to adjust a pH value of milk to pH 4.3 to 4.8 thereby to precipitate casein. However, the acid casein is inferior in hydrophilic property, and cannot be dissolved without the use of strong acidic or alkaline water. For this reason, the acid casein is rarely used in foodstuff, but is dissolved in a strong alkaline liquid to be used for industrial applications as adhesives or paper coating agents.
It is considered that intensive electrostatic interaction takes place among protein molecules together with formation of hydrophobic bondings during the acid precipitation process. It is known that the proteins linked by strong intermolecular bondings are not dispersed in water unless the water is strongly acidic or alkaline.
On the other hand, rennet casein is prepared by heating skim milk at 30.degree. to 37.degree. C., adding with rennin and coagulating in the presence of calcium ions, so that the major portion of protein binds with calcium. Thus, it is inferior in hydrophilic property. The rennet casein is used mainly as a plastics molding material for the production of buttons or the like, although a trial has been made to use the same for a cheese analogue. The co-precipitation protein product is prepared by adding calcium chloride to milk, followed by heating to 90.degree. to 95.degree. C., to co-precipitate casein with whey protein. Since the co-precipitation protein product is mainly composed of protein bound with calcium, it cannot be dispersed in water unless it is processed by the addition of 2 to 6 wt% of sodium polyphosphate. The products obtained by the enzymatic precipitation by rennin in the presence of calcium ions or by the co-precipitation by the use of calcium chloride are composed of proteins bound with calcium and are not expected to have water-dispersible and hydrophilic properties.
On the other hand, Japanese Patent Laid-Open Publication No. 20169/1979 filed by the assignee of this application proposes a process wherein milk is heated at pH 5.2 to 5.9 to a temperature of 50.degree. to 90.degree. C. under a static condition without being vibrated to prepare a soft curd edible diet of custard-like form. The edible diet obtained by this process is prepared by solidification of milk in its entirety in the form of custard-like curd containing milk protein and serum. Fox (P. F. Fox: Developments in Dairy Chemistry-I Proteins, p212, 1982, Applied Science Publishers) has reported that raw milk is coagulated by raising the temperature thereof by heating at pH 5.5 to 66.degree. C. under a static condition. However, this report only describes the physical change of the milk.